30.9.17

Scientists record a fourth set of gravitational waves

Last year, researchers confirmed the existence of gravitational waves with two Laser Interferometer Gravitational-wave Observatory (LIGO) detectors.
Shortly thereafter, they detected two additional
gravitational wave-causing events that sent ripples through the
universe. Well, we can now add a fourth to that list, as astronomers
announced another set of waves. And for the first time, they observed
the waves with a third detector -- the Italy-based Virgo.

Let's review a little before we dig into the huge benefits that
come with having three detectors rather than two. First, gravitational
waves are essentially ripples in spacetime. When some major event occurs
in the universe -- like, for example, when two black holes merge into
one -- a ripple is sent out in all directions and it travels through
spacetime at the speed of light. Albert Einstein predicted these sorts
of waves existed, but it wasn't until the LIGO project that researchers
could actually observe them.

The
LIGO and Virgo detectors are all largely the same design. Two very long
tunnels are arranged perpendicular to each other. At the point where
they meet, a laser beam is split and part of it travels down one tunnel,
and the other part down the second tunnel. Mirrors at the tunnel ends
bounce the beams back and if no major cosmic activity has occurred, the
two beams cancel each other out. However, if say two black holes slam
into each other and create gravitational waves, those waves will stretch
and pull spacetime, changing the length of the tunnels ever so
slightly. When that happens, the two laser beams are bounced back at
slightly different times and when they meet, the difference between them
provides astronomers with all sorts of information about what happened,
where and when.
The earlier detections of gravitational waves
were done with the two LIGO detectors in Washington and Louisiana. Virgo
joined them on August 1st and scored its first detection on August
14th. The gravitational waves that were detected were created by two
black holes -- 31 and 25 times the mass of our Sun -- merging around 1.8
billion light-years away. The resulting black hole is approximately 53
times the mass of the Sun. What happened to those three leftover solar
masses? They were converted into gravitational wave energy.
A
third detector means scientists can get a much better idea about which
direction the waves came from and it works similar to the way
seismometers pinpoint the location of an earthquake. The two LIGO
detectors themselves can provide a general direction of the event -- a
pretty large area equal to around 1/40th of the night sky. But adding
Virgo into the mix reduces the window to a tenth of that area, which
means once a signal is detected, astronomers can swing a telescope
towards the region of origin and maybe catch a glimpse of it in action.
The Virgo team hints in their press release that more detections from the three units will be announced sometime soon and some are hoping to hear that they've snagged a measurement, and maybe even a visual, of another big celestial event -- two neutron stars merging.
The recent detection was described in a paper published in Physical Review Letters.Image: NASA